Electrophotography in which the electrostatic latent image formed on the latent image bearing member such as a photosensitive member is developed with charged toner and an image is formed has become widespread. As the electrophotography of this kind, for example, a dry developing method directly using powdery toner and a wet developing method (liquid developing system) using the liquid developer in which the toner is dispersed in a liquid exist. Of these, in the liquid developing system, the toner is dispersed in the medium (carrier) liquid, and therefore, image formation can be carried out by controlling particles with a particle size in a submicron order, and the liquid developing system is a promising developing method in terms of high image quality and high definition.
In the wet developing method, image formation is carried out by moving toner particles contained in the liquid developer to media by electrophoresis. In the wet developing method, specifically, in an opposing portion of a film forming electrode provided opposed to a developing roller, a developer containing an appropriate amount of the toner is formed in a film (layer) on the developing roller and a toner layer is formed on the developing roller by a squeeze roller. In a subsequent migration process, i.e., in respective processes of development, primary transfer and secondary transfer, basically, movement of all the toner (particles) is an image formation principal. Accordingly, on a density of an image formed on media, an application amount of the toner in the liquid developer formed in the film on the developing roller is reflected. Therefore, stable control of the application amount of the toner in the liquid developer carried on the developing roller is very important because the stable control leads to stabilization of an image quality over a long term.
As an image forming apparatus in which the application amount of the toner on the developing roller is controlled at a certain value, for example, an image forming apparatus including an optical sensor capable of detecting a surface of the developing roller has been known (see Japanese Laid-Open Patent Application (JP-A) Hei 10-268645). In this image forming apparatus, a liquid developer formed in a film on the developing roller under a predetermined condition is irradiated with light, and reflected light thereof is detected by the optical sensor, so that a concentration of the liquid developer is measured. Then, an acquired result is fed back to control of toner and carrier liquid amounts, a charge control agent amount and the like in a developing liquid tank, so that the application amount of the toner on the developing roller is controlled. According to this image forming apparatus, a concentration itself of the liquid developer on the developing roller is measured and is capable of being utilized in feed-back control, and therefore, as long as the measured developer concentration is proper, it is possible to stabilize the concentration of the liquid developer on the developing roller.
However, in the image forming apparatus of JP-A Hei 10-268645, the surface of the developing roller is measured utilizing the optical sensor, so that there is a possibility that the following problem occurs. That is by use of the image forming apparatus for a long period, surface roughness of the developing roller increases with the use and reflected light intensity changes, and therefore, there is a problem such that in a measuring method using the reflected light, it is difficult to detect the toner concentration of the liquid developer on the developing roller for the long period with high accuracy. For this reason, in the above-described image forming apparatus, due to a lowering in detection accuracy of the toner concentration of the liquid developer on the developing roller by the use thereof for the long period, it becomes difficult to carry out stable control of the toner concentration of the liquid developer formed in the film on the developing roller during image formation.
The present invention aims at providing an image forming apparatus capable of detecting a toner concentration of a liquid developer carried on a developing roller for a long period with high accuracy in the image forming apparatus using a liquid developing system.